Currently, strings made from synthetic fiber monofilaments and/or multifilaments such as polyamide are used widely as strings for racquets used in sports such as tennis, badminton, and squash. Although important characteristics required of these racquet strings are playability and durability, basically these have an inverse relationship and there is a demand for a racquet string that satisfies both characteristics. For example, although racquet strings constituted by polyamide-based synthetic fiber multifilaments have excellent playability compared to natural gut, their durability is inferior when compared with monofilament synthatex strings, and there is a demand for this to be improved.
In JP H9-10367A, the applicant of the present invention has suggested that when a resin coating layer is present on the surface of racquet strings, the durability of the strings can be improved by including aluminum boronate whiskers, for example, in the surface resin coating layer. This has the effect of improving the durability to a certain extent, but, in order to give a sufficient durability improvement effect, issues remain such as the need to blend in a relatively large amount since the shape of the whiskers is fibrous and the particles are relatively large. Furthermore, when the blending amount is large, problems such as filter blockage, which may occur during melt coating, tend to occur. There is also the problem of a reduction in playability when the blended amount is large.
As noted above, conventionally it has been difficult to find a balance between playability and durability, and furthermore, even in the method of adding whiskers to the coating layer to improve durability that was suggested in JP H9-10367A noted above, the reality at present is that a string that can adequately satisfy the durability improvement effect, strike feeling and processability has yet to be found.